Police stand guard as they block access to the high street, Thursday, Aug. 11, in Tottenham, north London. Britain's riots began Saturday when an initially peaceful protest over a police shooting in north London turned violent. That clash triggered wider lawlessness that police struggled to halt. Across London, and then in cities throughout England, rioters set stores on fire and looted shops for sneakers, bicycles, electronics and leather goods.

“Everyone watching these horrific actions will be stuck by how they were organized via social media,” Mr. Cameron on told British lawmakers in an emergency session of parliament. “Free flow of information can be used for good. But it can also be used for ill. And when people are using social media for violence we need to stop them.”

The riots have brought Britain face to face with a trend that has emerged the US, albeit on a vastly smaller scale: "flash robs" – the criminal version of "flash mobs." Large groups of thieves have appeared suddenly at stores in cities from Las Vegas to Chicago to Washington, with law enforcement suspecting that raids were organized on social media.

But the scale of the chaos in England has prompted a far greater urgency, pitting law and order against the very social media tools that were hailed for supporting public demonstrations against authoritarian regimes during recent "Arab Spring."

In some cases, the calls to action among England’s disaffected have walked a fuzzy line between activism and incitement. After the shooting death of Tottenham resident Mark Duggan by police, some supporters organized a vigil Facebook page. But shortly afterward, the page had more than 7,500 fans and several dubious comments asking people to share videos from the riots.

Other posts, however, would appear to cross a clear line. One typical BlackBerry message broadcast on Sunday, reported by The Guardian newspaper, calls on "everyone from all sides of London" to convene in certain areas and then to vandalize shops on Oxford street.

This has ramped up pressure on social media providers to cough up the keys to the kingdom – the encryption keys that will let police or intelligence authorities listen in on private conversations.

But some say that would be missing the point.

"There’s certainly some evidence of the use of mobile communications to organize what happened, but to understand this you have to go beyond the assumption that this was a spontaneous public uprising," writes Andy Williamson, director of digital democracy at the Hansard Society in London, who has studied the use of social media tools in the Arab Spring.

"You can’t blame social media for what happened nor can you really say it changed the nature of the riots," he notes in an e-mail interview. "It might have spread the message but the evidence points to it being a tool.... These kids use social media instinctively in their lives, of course they’re going to use it here. If social media was to blame, 24-hour TV was more so."

Others say forcing providers to divulge encryption keys would be a knee-jerk reaction.

“The UK certainly has ample laws that would allow police to subpoena people's communications without going to BBM and having them open up their entire network," says Jillian York, director for international freedom of expression, Electronic Frontier Foundation in San Francisco

The BlackBerry Messenger (BBM) is particularly under fire because it is encrypted and free, Ms. York says. Many people can communicate at the same time, but it doesn't leave a public trail the way Twitter and Facebook do.

"I predict in the wake of London's riots that a whole lot of people will get arrested," he says. "This is a city filled with [surveillance] cameras. Police will examine the footage and correlate it with social media.”

Moreover, social media participation in England has not been all bad. It has also been an important part of the cleanup efforts.

"Basically you had people using tools like Facebook or Twitter to reach out and kind of say, ‘Hey, this is our community, let's get together to put a stop to this,’ " writes Patrick Underwood, a researcher of online communities at the University of Washington, in an e-mail interview. "Again, we've seen such a response in riots before, but the new technologies add some new wrinkles."